Co-worker receives email, sends note, no email or alert

When I assign my co-worker to a role, he is alerted through email. In that email, it allows him to “reply” with a comment. He replies with a comment, but it goes nowhere. I do not receive an email (as project owner) and nothing appears on the Asana site or in the task he commented on. Any thoughts?

Did he receive an error by email as a result? If not, and nothing appears in Asana, the error email is probably in the spam…

I checked, but not in SPAM/Junk either.